"Canonical has today publicly confirmed that they are working on a new cross-platform displayer server for Ubuntu. Called 'Mir', the X Window Server replacement is tasked with 'enabling development of the next generation Unity'. Which, in yet another about-turn, is to be rebuilt in Qt/QML." It'll be used for all Ubuntu variants (phone, tablet, desktop), and the first version will be released come May.

Ubuntu wants to become a desktop Android, a platform friendly for commercial, closed source software.
Present free desktop efforts failed at that, among others because the OS/Distro split that has marginalized Linux as a platform.
The situation you've described has defacto existed for commercial developer for years due to pointless distro incompatibilities on system level and lack of backward compatibility. Canonical is simply catering to the reality and sitting in the driver seat.
The remaining distros will remain as they are in the semi coherent free desktop world, while ubuntu might get to become a true platform. The free apps will eventually get ported to it.